AUDIE CORNISH, HOST: A top official with the U.S. Drug Enforcement Administration tells NPR efforts to target drug cartels in Mexico have unraveled. The culprit is a diplomatic route that's frozen joint investigations and intelligence sharing between the two countries. This comes at a time when cartels are shipping more and more fentanyl into the U.S., driving a record spike in overdose deaths. NPR addiction correspondent Brian Mann reports. BRIAN MANN, BYLINE: Last year overdose deaths in the U.S. surged in a way no one's ever seen, killing more than 90,000 Americans. Matthew Donahue is head of operations for the DEA, which means he directs U.S. efforts to curb drug trafficking around the world. He says illegal labs run by cartels in Mexico are the major source of fentanyl and methamphetamines driving the epidemic.